The present invention concerns compounds, compositions and methods for protecting crops from nematode attack. More specifically, the present invention is directed to compounds and compositions, which, when used as a dressing for seeds or tubers of a crop, or when applied directly to the crop or its locus, induce local and systemic resistance of the crop against attacks caused by soil-borne plant-parasitic nematodes. Such action is hereinafter defined as "systemic acquired resistance (SAR)".
Nematodes are a large group of parasites which attack both plants and animals. Plant-parasitic nematodes are obligate parasites, obtaining nutrition only from the cytoplasm of living plant cells. These nematodes are tiny round worms, about one mm long, that damage food and fiber crops throughout the world and cause billions of dollars in losses annually (Williamson and Hussey, The Plant cell 8: 1735-1745, 1996). Hereinafter, unless otherwise stated, the term "nematodes" will refer to soil-borne plant-parasitic nematodes.
Over 20% of the annual yield losses of major crops in the world occur from plant-parasitic nematodes: monetary losses due to nematodes is estimated to be US $100 billion worldwide. There is a correspondingly large market for nemacides, or agents which kill nematodes and suppress nematode infections.
Most of the economic damage is caused by the sedentary endoparasitic nematodes of the family Heteroderidae. This family is divided into the cyst nematodes (Heterodera and Globodera) and the root-knot nematodes (Meloidogyne). The root-knot nematodes, so-called for the root galls (root knots) that they form on many hosts, infect thousands of plant species and cause severe losses in yield of many crops (vegetables and fruits) as well as flowers and other ornamentals. Symptoms of diseased plants include stunting, wilting and susceptibility to other diseases. During the infection process, elaborate developmental and morphological changes occur in host root cells especially in those become the feeding cells that provide the sole source of nutrients for the nematode. The life cycle of the root-knot nematode has a series of four stages. In the infective stage the nematodes are mobile, second-stage juveniles (J2) which emerge from the egg and find a host. After finding a host, each of the J2 nematodes penetrates the root. Then, the J2 nematode migrates to the root tip and finally proceeds to the differentiation zone where procambial host cells adjucent to the head of the nematode develop into "giant cells". These large multinucleate metabolically active cells serve as a permanent source of nutrients for the endoparasite (Williamson and Hussey, The Plant Cell 8: 1735-1745, 1996). Concurrent swelling and division of cortical cells around the nematode lead to the formation of galls typical of Meloidogyne spp.
After feeding is initiated, the nematode becomes a sedentary adult female following three molts which occur approximately 11 through 16 days after the initiation of feeding. After the last molt, the adult female continues to grow until it becomes a non-motile, saccate, egg-laying female. Egg production begins 3-5 weeks after the infection starts; eggs are released and the infection cycle may start again. Males regain motility during the third molt before leaving the root.
Chemical treatment of the soil is one of the most promising means to control plant-parasitic nematodes. This control method depends upon bringing the nematode-toxic chemicals into contact with the nematode in sufficiently high concentrations. Such contact may be accomplished by percolation of the chemicals in water or a gaseous diffusion of a nematocide in the soil. The nematocide must penetrate the nematode body in order to kill it, either through the cuticle or body openings, or during feeding.
Methyl bromide, organophosphates and carbamates are widely used nematocides, which unfortunately are highly hazardous to the enviroment. Organophosphates and carbamates paralyze the nematode by inhibiting acetylcholinesterase enzyme activity which is essential for neural activity (The Pesticide Manual, 10th Edition, 1994, Crop Protection Publications, 1341 pp.).
Another way to control nematodes is by using genetically-resistant cultivars. Plants are defined as resistant to nematodes when they are able to reduce the level of nematode reproduction (Trudgill, Ann. Rev. Phytopathol 29: 167-193, 1991). Nematode resistance genes are present in several crop species such as tomato, potato, soybean and cereals. With many of these resistance genes, a localized necrosis or hypersensitive response, resembling that described for other pathogens resistance genes, is associated with nematode infection. One such resistance gene is the Mi in tomato which confers resistance against root-knot nematodes (Williamson and Hussey, The Plant Cell 8: 1735-1745, 1996). However, this resistance gene was recently reported to be unstable (Kaloshian et al., Cal. Agri., 50:18-19, 1996).
A third strategy is to induce plant resistance against pest organisms by adding various chemicals to the plants themselves. While the strategy of inducing systemic acquired resistance (SAR) has been extensively used by commercial companies to protect plants from fungal attack, no such strategy has been used against nematode attack. Systemic acquired resistance (SAR) in plants is gained by either a primary inoculation with an incompatible pathogen (virus, bacterium, fungus) that induces a hypersensitive local lesions in the host, from which a signal is translocated to untreated parts of the plant so as to protect them from diseases caused by fungi, bacteria or viruses, or by using a chemical which has no direct effect on plant pathogenic organisms, but rather can induce a SAR response (Ryals et al--Plant Cell, 8: 1809-1819; 1996).
SAR in a crop is manifested by various defence mechanisms including the accumulation in the crop of soluble proteins referred to as pathogenesis-related (PR) proteins. Some PR-proteins have been shown to be hydrolytic enzymes such as chitinases and .beta.-1,3 glucanases, while others are shown to be peroxidases. Also accumulated is a group of proteins having a molecular weight of about 10 to 20 kDaltons which are called P14 proteins, which are also known to possess antifungal function. All of these proteins are believed to participate in the defense system of a crop.
Various isonicotinoyl-pyridinyl-hydrazin-derivatives and benzothiadiazole compounds have been described in patent literature as immunizing healthy plants against fungal diseases (European Patent Applications 0 268 775, 0 288 976 and 0 313 512).
Another class of molecules is DL-3-aminobutyric acid (BABA) and derivatives which have been shown to induce local and systemic resistance against fungal attack (Cohen et al., Plant Physiology 104: 59-66, 1994; Cohen, Phytopathology 84: 55-59, 1994; Cohen, Physiol. Molecular Plant Pathol. 44: 273-288). The use of BABA against fungal attack in crop plants has been described in several patent applications: Israel Application No. IL107,992; Israel Application No. IL109,474; Israel Application NO. IL111,828; PCT Application No. US 94/14,108; PCT Application No. WO95/15, 684. However, these compounds have never been examined for their efficacy against species of plant-parasitic nematodes.
Little information is available on the induction of plant resistance against nematodes. Ogalo and McClure (Phytopatholgy 86: 498-501, 1996) reported that a prior inoculation of tomato roots with the incompatible nematode Meloidogyne incognita induced resistance against the compatible nematode M. hapla. However, no direct induction with a chemical composition has been demonstrated.
It would therefore be highly useful to have a chemical composition which can induce plant resistance to nematodes when applied directly to the whole plants or a portion thereof